A driver driving a vehicle such as an automobile is required to grasp the state of the outside of the vehicle, read information of a display device of the vehicle, and drive the vehicle safely and speedily during driving. In view of this it is desirable to provide a configuration such that information of a display device of a vehicle is readable in a range necessary for grasping the state of the outside of the vehicle during driving. For instance, it is expected to implement an image display device for displaying characters or images by causing light to irradiate onto a part of a transparent plate such as a front glass of a vehicle.
As examples of the transmissive image display device, there are known a head up display (hereinafter, called as “HUD”) for displaying driving information on a front glass of an automobile, and a head mounted display (hereinafter, called as “HMD”) for displaying information on a lens portion of eyeglasses. Use of the transmissive image display device allows for the driver to see the information (e.g. a map or a speed meter) relating to driving while visually recognizing the outside world. Thus, it is expected that the driver can more safely drive the vehicle.
As the conventional HUD, there is known an image display device for projecting a virtual image onto a front glass (see patent literature 1). FIG. 25 illustrates an example of a conventional HUD for projecting an image onto a front glass.
In the example of FIG. 25, a HUD housing 100 is accommodated in a dashboard of a car. A light source 101, an incident optical system 102, a display element 103, and a reflection mirror 104 are disposed in the HUD housing 100. The light source 101 is a light source such as a laser or an LED (Light Emitting Diode). The light source 101 illuminates the display element 103 through the incident optical system 102.
The display element 103 is a two-dimensional display element such as a liquid crystal panel or a DMD (Digital Mirror Device). The display element 103 receives light from the light source 101 for forming display light, and outputs the display light to the reflection mirror 104. The display light from the display element 103 is deflected on the reflection mirror 104, and is incident on a front glass 106 through an opening 105 formed in the HUD housing 100.
In a general HUD, it is often the case that the reflection mirror 104 employs an element having a function of optically enlarging an image such as a concave surface mirror in order to enlarge an image to be visually recognized by the driver. Display light 109 reflected on the front glass 106 is incident on the eyeball of a driver 108. Then, the driver 108 can visually recognize a virtual image 107 afar from the front glass 106.
In the specification, the area where the display light 109 reaches is called as an eye box 110. When the eyeball of the driver 108 lies in the eye box 110, the driver 108 can visually recognize the virtual image 107. Use of the above configuration allows for the driver 108 to visually recognize information necessary for driving, without the need of largely moving the line of sight even during driving.
However, in the HUD having the configuration as illustrated in FIG. 25, due to the large HUD housing 100, it may be difficult to secure a sufficient capacity in the inside of the dashboard. When the HUD housing 100 is large, it is difficult to load the HUD housing 100 in a compact car. Further, when the opening 105 is large, the external appearance of the dashboard may be impaired. It is desired to provide a HUD with a small housing and with a small opening. In order to satisfy the above demand, there is proposed an optical system including combination of a light guiding plate and a diffraction element (see e.g. patent literature 2 and patent literature 3).
FIG. 26 illustrates a conventional example of a HUD provided with an optical system including combination of a light guiding plate and a diffraction element. In the example of FIG. 26, a HUD housing 200 is accommodated in a dashboard. A light source 201, an incident optical system 202, a display element 203, an incident diffraction element 204, and a part of a light guiding plate 205 are accommodated in the HUD housing 200. The light source 201, the incident optical system 202, and the display element 203 are substantially the same as those in FIG. 25 and accordingly, description thereof is omitted.
Display light 207 from the display element 203 is incident on the incident diffraction element 204 provided in the light guiding plate 205. The light guiding plate 205 guides the display light 207 by total reflection therein. Further, the light guiding plate 205 is made of a material capable of transmitting light from the outside world, for instance, is made of a transparent material such as glass or acrylic resin in order to prevent obstruction of the view of a driver 210.
The incident diffraction element 204 has a function of changing the angle of incident light so that the incident display light 207 causes total reflection in the light guiding plate 205. As an example of the incident diffraction element 204, it is possible to use a diffraction element such as a volume hologram or a relief hologram. The display light 207 is incident on an output diffraction element 206 provided in the light guiding plate 205, while repeating total reflection in the light guiding plate 205.
The output diffraction element 206 acts on light incident at a specific incident angle. In the example of FIG. 26, the display light 207 is incident on portions indicated by the broken lines 211a, 211b, and 211c at an angle at which light is diffracted on the output diffraction element 206. Accordingly, the display light 207 is diffracted at the portions indicated by the broken lines 211a, 211b, and 211c, and diffraction light 208 is generated.
Further, the output diffraction element 206 defines the travelling direction of the diffraction light 208 so that the diffraction light 208 is output from the light guiding plate 205. In this example, the output diffraction element 206 is designed so that the angle of reflection of the diffraction light 208 on the light guiding plate 205 is not larger than the total reflection angle on the light guiding plate 205. The above operation allows for the display light 207 subjected to total reflection in the light guiding plate 205 to be output from the light guiding plate 205 as output light.
In FIG. 26, the output diffraction element 206 is designed to diffract a part of incident light and to transmit a part of the incident light. Accordingly, a part of light causes total reflection in the light guiding plate 205, and is diffracted at a position (the portions indicated by the broken lines 211b and 211c in FIG. 26) where the light satisfies the diffraction condition of the output diffraction element 206, even after the display light 207 is diffracted on the portion indicated by the broken line 211a in FIG. 26.
As described above, adjusting the diffraction efficiency of the output diffraction element 206, and generating diffraction light at a plurality of positions on the output diffraction element 206 makes it possible to increase an area where the driver 210 can visually recognize the diffraction light 208. The above configuration is advantageous in increasing an eye box 212 (an area where the driver 210 can visually recognize a display image).
As described above, allowing the diffraction light 208 to be incident on the eyeball of the driver 210 allows for the driver 210 to visually recognize a virtual image 209 afar from the light guiding plate 205.
In the example illustrated in FIG. 26, use of an optical system including combination of the light guiding plate 205 and the output diffraction element 206 makes it possible to reduce the capacity for a reflection mirror or the like necessary in the example illustrated in FIG. 25, thereby reducing the capacity of the housing 200. Further, it is only necessary to secure the thickness of the light guiding plate 205 as the size of the opening necessary for the dashboard. Thus, it is not necessary to form a large hole in the dashboard. This makes it possible to provide a HUD with less design impairment on a car.
However, in the conventional HUD as described above, there is no measures against an influence of diffraction of unwanted light (e.g. external light).